Fault currents motor contribution

[hhsting]

In the attached sketch their are fault current locations labeled F1,F2,F3,F4,F5 and F6.

I am wondering at location F1, F2,F3,F4,F5,F6 does the motor contributions of M1,M2,M3,M4 need to be taken into account or not? If not then which motors’ motor contribution would need to be taken into account at each of the fault current location points?

 

[jim dungar]

We have been through this before.
All fault currents must be included in the analysis. However for small motors their individual contribution may be minimal, which is why some study short cuts ignore motors less than 50hp.
Motors must be running to create fault current, however their actual load is not important.
One hundred 1hp motors don't produce the same amount of fault current as does a single 100hp unit.
Series ratings are based on limited motor contribution being inserted between the two devices in series, as would occur on the bussing of a panelboard.

 

[hhsting]

We have been through this before.
All fault currents must be included in the analysis. However for small motors their individual contribution may be minimal, which is why some study short cuts ignore motors less than 50hp.
Motors must be running to create fault current, however their actual load is not important.
One hundred 1hp motors don't produce the same amount of fault current as does a single 100hp unit.
Series ratings are based on limited motor contribution being inserted between the two devices in series, as would occur on the bussing of a panelboard.

I understand all fault currents including motor contribution.

I was looking at the following in link page 238 system A where their is motor. However motor contributions are Not taken into account fault location 1,2 only at 3 below link.

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-electrical-formulas.pdf

So that got me wondering and I am just not sure which motor contributes at locations F1,F2,F3,F4,F5,F6? Do M1,M2,M3,M4 all of them do at all locations or only some do and which ones?

 

[jim dungar]

That link is showing a simplified short circuit current method, which employs many shortcuts. Ignoring motor contribution is just one of them.

 

[hhsting]

That link is showing a simplified short circuit current method, which employs many shortcuts. Ignoring motor contribution is just one of them.

I dont follow. So then back to post #1 question

Should F1 locations include M1 to M4 motor fault current contributions or only some of the motors?

Same question for F2 to F6?

 

[EC Dan]

I understand all fault currents including motor contribution.

I was looking at the following in link page 238 system A where their is motor. However motor contributions are Not taken into account fault location 1,2 only at 3 below link.

https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/fuses/solution-center/bus-ele-tech-lib-electrical-formulas.pdf

So that got me wondering and I am just not sure which motor contributes at locations F1,F2,F3,F4,F5,F6? Do M1,M2,M3,M4 all of them do at all locations or only some do and which ones?

In that Eaton link, motor contribution IS taken into account at all three points. Isc = 7217 A and is included at all three points. Study that system A diagram again and see how motor contribution is added to each fault location AFTER the short circuit current is sequentially reduced based on conductor runs.

 

[hhsting]

That link is showing a simplified short circuit current method, which employs many shortcuts. Ignoring motor contribution is just one of them.

I still don’t follow how is the link follow shortcuts ignoring motor contribution? It adds motor controbutions

 

[xptpcrewx]

I still don’t follow how is the link follow shortcuts ignoring motor contribution? It adds motor controbutions

Because the procedure provided in the Eaton link is a kindergarten version of short-circuit analysis.

 

[jim dungar]

I still don’t follow how is the link follow shortcuts ignoring motor contribution? It adds motor controbutions

That particular link includes some type of motor contribution in their example, but it does not show how they decided on those values. It also says you could have used a different motor contribution if the motor loading was different. All in all it's not helpful except to show motor contribution must be included.

My comment was this link is an example of a short circuit analysis using short cuts. One typical short cut is to exclude motors. Another common short cut ignores X/R ratios.

 

[hhsting]

That particular link includes some type of motor contribution in their example, but it does not show how they decided on those values. It also says you could have used a different motor contribution if the motor loading was different. All in all it's not helpful except to show motor contribution must be included.

My comment was this link is an example of a short circuit analysis using short cuts. One typical short cut is to exclude motors. Another common short cut ignores X/R ratios.

Motors are not excluded so I dont follow. First page 267 where it says point to point calculation step 6A it says 4 to 6x motor FLA for motor contribution fault current.

I thought that is typical values derived from ANSI or some other codes when you dont have x/r ratios or motor not know?

 

[xptpcrewx]

Motors are not excluded so I dont follow. First page 267 where it says point to point calculation step 6A it says 4 to 6x motor FLA for motor contribution fault current.

I thought that is typical values derived from ANSI or some other codes when you dont have x/r ratios or motor not know?

As mentioned, its complicated. The point-to-point calculation is just a loose estimate. Most project specifications prohibit the point-to-point method for this reason. If you really want to get into the details, you can reference ANSI/IEEE C37.010, C37.5 and C37.13.

​

 

[jim dungar]

Motors are not excluded so I dont follow. First page 267 where it says point to point calculation step 6A it says 4 to 6x motor FLA for motor contribution fault current.

I thought that is typical values derived from ANSI or some other codes when you dont have x/r ratios or motor not know?

If you guess at the inputs to your short circuit study, you may end up just guessing at the resultant values.
This isn't always bad, just be aware of what you are doing.

For example, are you going to pick 4 or 6 times motor full load amps for your input? the results can differ by 150%.
Are you going to use the NEC table amps, typical full load amps, or the actual nameplate values (the actual motor loading is not important).

Decisions like this are why many specifications require a trained individual or liicensed engineer, to provide a detailed short circuit study as part of the project deliverables.

 

[hhsting]

If you guess at the inputs to your short circuit study, you may end up just guessing at the resultant values.
This isn't always bad, just be aware of what you are doing.

For example, are you going to pick 4 or 6 times motor full load amps for your input? the results can differ by 150%.
Are you going to use the NEC table amps, typical full load amps, or the actual nameplate values (the actual motor loading is not important).

Decisions like this are why many specifications require a trained individual or liicensed engineer, to provide a detailed short circuit study as part of the project deliverables.

But that is what was provided by the electrical designer point to point method. He chose 4x full load. Their is AC, heat pump for dwelling units motors, elevator motors.

If point to point is bad then what is good? X/R ratio and also I think ANSI C37.010 provides guidance as to what to chose. It says induction motors 75% are 3.6 times motor contribution and synchronous motors 25% are 4.8 times for motor contribution. No?

 

[xptpcrewx]

But that is what was provided by the electrical designer point to point method. He chose 4x full load. Their is AC, heat pump for dwelling units motors, elevator motors.

If point to point is bad then what is good? X/R ratio and also I think ANSI C37.010 provides guidance as to what to chose. It says induction motors 75% are 3.6 times motor contribution and synchronous motors 25% are 4.8 times for motor contribution. No?

See post #11.

 

[hhsting]

See post #11.

That table references low voltage, and medium to high voltage and multipliers in terms of x not full load amps?

 

[jim dungar]

But that is what was provided by the electrical designer point to point method. He chose 4x full load. Their is AC, heat pump for dwelling units motors, elevator motors

Then why are you questioning it?

Designers often use simplified techniques and then specify a through study as part of the project contract.

 

[hhsting]

Then why are you questioning it?

Designers often use simplified techniques and then specify a through study as part of the project contract.

I did not see any through study as part of project contract being specified. But will ask the electrical designer

 

[hhsting]

As mentioned, its complicated. The point-to-point calculation is just a loose estimate. Most project specifications prohibit the point-to-point method for this reason. If you really want to get into the details, you can reference ANSI/IEEE C37.010, C37.5 and C37.13.

View attachment 2557558​

What is the range of LV, medium and high voltage?

 

[hhsting]

Please see link document below:

https://www.ee.co.za/wp-content/uploads/legacy/Vector%202013/vector_june2013_ge.pdf

The link above says the following:


ANSI standard C37.010 offers guidance when calculating motor contribution for a group of LV motors if detailed motor data is not available. Assuming a motor contribution of four times rated full load current is acceptable. The standard arrived at this value by assuming that the motor contribution of 3,6 times rated current came 75% from induction motors and 4,8 times rated current from 25% synchronous motors…..


Does anyone know or have link where to find this in ANSI C37.010? Post #11 does not spell out 4x full load amps

 

[don_resqcapt19]

Please see link document below:

https://www.ee.co.za/wp-content/uploads/legacy/Vector%202013/vector_june2013_ge.pdf

The link above says the following:


ANSI standard C37.010 offers guidance when calculating motor contribution for a group of LV motors if detailed motor data is not available. Assuming a motor contribution of four times rated full load current is acceptable. The standard arrived at this value by assuming that the motor contribution of 3,6 times rated current came 75% from induction motors and 4,8 times rated current from 25% synchronous motors…..


Does anyone know or have link where to find this in ANSI C37.010? Post #11 does not spell out 4x full load amps

If you need real numbers, you need someone to run the system in one of the electrical engineering software programs...everything else is just a good guess.